Electrodepositing manganese



July 2, 1957 R. s. DEAN 2,798,038

ELECTRODEPOSITING MANGANESE Filed Dec. 2, 1953 fiegrees 7M JMVENTORELECTRQDEPSSITKNG MANGANEE Reginaid S. Dean, Hyattsville, Md.

Application December 2, 1953, Serial No. 395,367

9 Claims. ((11. 204-45} This invention relates to the electrodepositionof manganese. It relates especially to processes for producingelectrolytic manganese from pure manganous oxide or manganous carbonate.The invention has for its principal object the deposition of manganesefrom solutions of manganese salts containing pure manganous carbonate oroxide or other acid-soluble oxidic compound of manganese yielding a puremanganous salt by reaction with dilute acid in a state of suspension insuch solutions. It also has for its aim the production of electrolyticmanganese in a single compartment cell.

In the heretofore practiced art of electrodepositing manganese, it hasbeen the practice to use a diaphragm cell. The reason for this practicewill be clear from a consideration of the necessary conditions fordepositing manganese. Since manganese is less noble than hydrogen, itcan only be deposited from solutions of low hydrogen ion concentration,that is, from alkaline solutions. In the electrolysis of manganesesolutions, the electrolyte immediately adjacent to the cathode becomesalkaline, and the electrolyte adjacent to the anode becomes acid. Theoverall cfiect, however, in electrodepositing manganese, is to free oneequivalent of acid for each equivalent of manganese. The necessaryalkalinity at the cathode can accordingly be maintained only bypreventing complete diffusion of the acid, formed at the anode, into thecathode area. This has been accomplished in the known art by theinterposition of a diaphragm between anode and cathode. Under theseconditions, the alkalinity at the anode may be allowed to rise to almostany desired point. Since manganese salts are precipitated at a pH near7, ammonium salts are usually added to prevent this precipitation.

It has also been proposed to prevent the diffusion of the acid, formedat the anode, to the cathode area by a rapid flow of solution throughthe cell, and also by the addition of ammonia to the cell. It will beobvious that these expedients are not practical because, in the firstinstance, only a very little manganese can be removed from a givenvolume of solution, and replenishment and readjustment of pH to theoriginal condition becomes uneconomical. In the second instance, theaccumulation of ammonium salts in the circuit eventually interferes withthe process.

I have found that the difiusion of the acid formed at the anode may beconveniently controlled by the presence in the anode area of suspendedparticulate manganous oxide or manganous carbonate, or other oxidiccompound of manganese yielding a pure manganous salt by reaction withdilute acid. These compounds not only serve to control acid ditfusionbut at the same time they replenish the manganese content of theelectrolyte. If these compounds are pure, they do not interfere with thedeposition of manganese. I have further found that the presence of thesesolid compounds, suspended in the electrolyte, does not in any waydisturb the deposition of manganese; consequently, they may be suspendedin the electrolyte which flows over the cathode. The evolution ofhydrogen, which always accompanies the deposition of manganese, preventsthe inclusion of any of the suspended solids in the cathode deposit.

It will be clear that these discoveries concerning the behavior of asuspension, in the electrolyte, of particulate acid-soluble oxidiccompounds of divalent manganese, more particularly pure manganous oxideor manganous carbonate, lead to many important improvements in the,process of electrodepositing manganese.

The pure manganous oxide and manganous carbonate of my invention may beprepared in many ways known in the art. I prefer to make manganouscarbonate in accordance with my invention described in U. S. Patent No.2,608,463. Pure carbonate may be converted to 0xide, in Whole or inpart, by heating, and the use ofpartially decomposed carbonate is withinthe scope of my invention, as is also the use of manganous hydroxide,hydrated manganous oxide, or any other oxidic manganese compound whichyields a pure manganous salt by reaction with dilute acid.

Perhaps the simplest embodiment of my invention is the suspension ofpure manganous carbonate in the usual electrolyte for the preparation ofelectrolytic manganese, namely, a solution containing about 16 grams perliter of manganese as sulphate, 100 grams per liter of ammoniumsulphate, and about .1 gram per liter of sulphite ion. If such asuspension is passed into a cell having a stainless steel cathode and alead-silver anode,

and the flow of solution adjusted so that it leaves the cell at a pH ofabout 4.5, deposition may be carried on continuously, and with the sameefiiciency as in a diaphragm cell. The amount of carbonate in thesuspension may be conveniently adjusted so that the solution leaving thecell will, on agitation, become substantially clear at pH 6.0. Underthese circumstances, the solution may, if desired, be purified by theaddition of sulphide ion, in accordance with the known art, beforepreparing the suspension of manganese carbonate for reuse. It will beevident, from this simple example, how my invention may be used toobviate the necessity of using in the cell a diaphragm with all itsattendant disadvantages.

My invention, however, goes beyond the mere elimination of the diaphragmfrom the usual electrolytic manganese cell. In an entirely general waymy invention permits a wide control over the acidity developed at theanode. It likewise permits a more precise control over the alkalinitydeveloped at the cathode. These features of my invention enable me touse electrolyte compositions, and conditions of deposition, notpreviously possible because of the inconsistency of anode and cathodeconditions necessary without the practice of my invention.

An outstanding illustration of the practice of my invention, toaccomplish the type of result just referred to, is the deposition ofmanganese from a suspension of pure manganous carbonate in anelectrolyte comprising manganous chloride and ammonium chloride. In theknown art, chloride electrolytes have been found to have manyadvantages, but their advantages have been overbalanced by the evolutionof chlorine at the anode, and by the oxidation of the ammonium salts inthe electrolyte at the anode. I have found that the presence of asuspension of manganese carbonate (or, manganous oxide) around theanode, whereby to increase the pH on the anode area to about 3.0,prevents the undesirable anode reactions mentioned. This may beaccomplished by adding manganous carbonate or oxide to the anolytecompartment of a conventional cell, having a graphite anode. A morefavorable result, however, may be obtained by simply passing thesuspension at a pH of about 6.0 through a single compartment cell, andadjusting the rate of flow so that the exit pH is about 4.5.

The conditions just described cannot, of course, be establishedindependently of other variables. The pH Patented July 2, 1957 gradientfrom anode to cathode, which is highly critical in any method ofelectrodepositing manganese, is a function of the rate of solution ofthe suspended particles of manganese carbonate (or its equivalent). Thisdepends on the physical nature of the suspended carbonate, its totalamount, the temperature of the electrolyte, and turbulence, which is inturn related to current density and the flow pattern of the electrolyte.All of these factors are in turn related to the primary platingconditions, solution flow, and current density. These primary factorsdetermine the amount of acid per unit volume which must be neutralizedby the carbonate in unit time.

In order to most fully explain my invention, as it applies to anelectrolyte of ammonium chloride and manganese chloride, the followingdata are given. The preferred concentration of manganese in such anelectrolyte is 12 grams per liter, but good results can be obtained with618 grams per liter. The ammonium chloride concentration is preferably125 grams per liter but good results are obtained with 100-150 grams perliter. The amount of manganese carbonate suspended in the electrolytewill depend on its physical nature, in a manner which will be discussedlater.

"I will first illustrate the relationship of current density to thenature of the electrodeposit, in a process where the enteringelectrolyte has a pH equal to 6.2 and the exit electrolyte has a pHequal to 4.5. Under these conditions the pH at the cathode is about 8.0,and at the anode 3.0. These are the preferred conditions at anode andcathode, respectively. In the experiments now to be described I havevaried the flow rate of the electrolyte and the amount of carbonate insuspension so as to achieve these conditions, regardless of currentdensity and temperature.

In the graph, Fig. 1, I have shown the relationship of current densityand temperature to the type of electrodeposit obtained, and haveindicated the combinations of temperature and current density, whichgive the best results for long time plating as required forelectrowinning manganese.

It should be pointed out that the control of pH varies greatly with thephysical nature of the carbonate. I prefer to use a coarse fullycrystalline carbonate, since this .dissolves slowly, and permits thecontrol of the exit pH by flow rate with wide differences in the amountof suspended carbonate. If the carbonate is very fine, it becomes morediflicult to control the exit pH.

Various features which enter into consideration in establishing andmaintaining good plating conditions will now be mentioned.

The point at which the suspension is added to the body of electrolyte inthe cell is important. If it is introduced directly adjacent the anodeneutralization is too rapid and the pH at the cathode becomes undulyhigh for good plating. If the suspension is introduced directly adjacentthe cathode the pH which would have to be established at the anode inorder to give good cathode plating conditions is too low. The mostfavorable condition is to introduce the suspension approximatelycentrally between anode and cathode.

Other variables having an influence on the efficiency of plating are (a)the geometry of the cell and (b) the size of the hydrogen bubbles formedat the cathode. It is important that these factors be so controlled thatturbulence is minimized. This is accomplished by having the depth of thecell in proper relation to the electrode spacing. If the depth is toogreat the hydrogen evolved near the bottom of the electrode will have along path, with expanding bubbles, and will create too much turbulence.In general, it has been observed that in the carrying out of theinvention minimization of turbulence is favorably influenced by makingthe cell shallower than conventional. The addition of wetting agentstothe electrolyte, to decrease the size of the hydrogen bubbles, is alsohelpful in minimizing turbulence.

Control of turbulence can be accomplished, further, by the imposition ofa suitable baffie or bafiles between anode and cathode. These bafflesare in no sense equivalent to the conventional diaphragm as theirpurpose is not to interfere with thermal convection or diffusion butrather to interfere with the setting up of gross eddy currents by theevolved hydrogen. These bafiies can be perforated plastic plates orplastic screens and can be spaced between the electrodes. The size andnumber of openings (interstices) must be so selected as not to disturbthe uniformity of the current flow between the electrodes.

In the discussion, supra, of the adherency of the manganese plate to thecathode it has been pointed out that at very high, or very low,temperatures the plate formed is unusually adherent to the cathode body.Advantage may be taken of this phenomenon in the preparation ofadherently plated objects. This adherency is general to all metalliccathodes but is particularly important for such metals as titanium,zirconium, molybdenum, magnesium, aluminum, and alloys thereof, whichhave heretofore not been satisfactorily plated. Such adherent manganeseplatings, on the metals mentioned, can be used as under-coats for topplatings of copper, nickel, or other desired electroplatable metal.

With the chloride electrolyte which I prefer, and which I have so farused to illustrate my invention, it is preferred to use a graphiteanode. This anode may be in plate form; or, a multiplicity of rods maybe used to decrease current density at the anode with relation to thecathode. This may be desirable where high cathode current densities areused, and if the same current density were used at the anode, chlorinemight be formed. Various methods of sculpturing an anode plate may beused to increase surface and hence to lower the current density.

Other anodes than graphite may be used; for example, the porous titaniumanode of U. S. Patent No. 2,608,531. I have found that with eithergraphite or porous titanium anodes the current density must not exceedabout amp. sq. ft. at pH=30, and about amp. sq. ft. at PH=40.

The cathode may be stainless steel, but I prefer to use titanium inaccordance with my previous invention, as disclosed in U. S. Patent No.2,646,396.

While I have described my invention in terms of its preferredembodiment, using manganese carbonate and an electrolyte of manganeseand ammonium chlorides, my invention may also be applied to otherelectrolytes, and to the use of manganous oxide and other acid-solubleoxidic compounds of manganese.

In selecting other electrolytes, consideration must be given tosecondary anode reactions. We have seen that the secondary reaction atthe graphite anode in a chloride electrolyte may be controlled by aproper selection of current density and pH.

In several electrolytes, the manganese is oxidized at the anode and thismust be minimized by a suitable selection of anode. Thus, in the case ofsulphate electrolytes, the anodes known in the art, such as lead-silver,may be used. I have found that when my invention is applied to sulphateelectrolytes, the current densities which may be used are greater thanthose of the conventional diaphragm cell process and may be as high asor higher than 75 amp. sq. ft. The ability to use higher currentdensities with my process is one of its great advantages.

' The reason for this advantage of my process is to be found in thecontrol of diffusion from anode to cathode. In the conventional processthe upper limit of current density is established by the alkalinity atthe cathode. This is controlled only by solution flow as difiusion ofacid from the anode is effectively prevented by the diaphragm. In theprocess of my invention both flow and anode acidity may be controlled,and the upper limit of current density is set by the nature of thedeposit.

Other electrolytes which may be used with my invention include:fluoborates, fluosilicates, sulphamates, alkylsulphonates,alkylsulphates, acetates, and hydroxyacetates of manganese, andmanganesesalts of. other acids forming soluble manganese compounds.

I have found that in all of these electrolytes, the best range ofconcentration of manganese is from 6 to 18 grams per liter, and the bestconcentration of ammonium salt is 100-140 grams per liter.

All of the above acids require the use of a graphite or porous titaniumanode. The cathode may be stainless steel ortitanium.

The addition agentswhich have been found useful in the heretofore knownart of plating manganese may be advantageously used with my process.-- Ihave found that sulphites are a useful addition agent. I prefer that thesuspension of manganese carbonatebe in an electrolyte saturated withmanganese sulphite. Any excess of manganese sulphite simply. passesthrough the cell since it is less soluble at low pH. Manganese sulphiteis more soluble at low temperatures, and when plating at lowtemperatures an excess of manganese sulphite suspended in theelectrolyte is to be avoided to prevent sulphur entering the plate.

Sulphide ion may be used in my process as an addition agent much moreeffectively than in heretofore known processes, since it may beintroduced as a suspension of manganese sulphide and is hence brought tothe plating zone in highly effective form.

Since sulphide ion may be so effectively used in my process sulphiteaddition may be omitted.

Other addition agents may be added such as hydroxylamine salts,thiourea, thiocyanate, and dithionates.

Since my process uses pure manganese carbonate or r'n'anganous oxide forpH control and replenishment of the electrolyte, the problems ofpurification which beset the conventional process are not met. However,should impurities enter the cycle by inadvertence, their accumulation todamaging proportions may be readily prevented. The electrolyte ifneutralized to pH 6.0 by manganese carbonate or oxide and filtered willbe purified from iron, since the iron will have been oxidized to theferric state at the anode. The electrolyte is then treated with NHa andCO2 and a little HzS to dissolve the manganese. Heavy metals will beprecipitated as sulphides and may be removed by filtration.

The resulting solution of carbamates may be warmed to 70 C. and themagnesium and calcium which will have accumulated in the electrolytewill be precipitated as car bonates and may be removed by filtration.The resulting solution may be used to neutralize the exit electrolytefrom the cell thus replenishing manganese and any ammonia lost from thesystem.

The solution will accumulate sulphate ion by oxidation of sulphide orsulphite. This may be removed by adding barium chloride and filteringthe barium sulphate sopre- I cipitated.

Having described my invention in its general aspects, I will nowillustrate it by specific examples.

Example 1 Type of cell Diaphragm. Electrolyte:

Acid ion Chloride. Mn g./l 12. NH4 salt g./l 100 NHCl. Suspended solid:

Chemical nature MnCOs. Physical nature 5 microns. Amount g./l.

Out 5.'

In 6.0. Out 4.5.

Example 1.Continued Additions .2 g./l. sulphite ion. Plating time 2400amp. hrs/sq. ft. cathode. Current density:

Anode amps. per sq. ft. Cathode 50 amps. per sq. ft. Deposit:

Character Smooth.

Efiiciency 70%. Adherence Easily stripped.

Anode behavior: No measurable chlorine evolution at graphite anode.Notes: Thick carbonate slurry added to anode compartment. Titaniumcathode.

Example 2 Type of cell Single compartment. Electrolyte:

Acid ion Sulphate.

Mn g./l 16.

NHs salt g./l '140. Suspended solid:

Chemical nature Manganese carbonate.

Fhysical nature 5 microns.

Amount g./l.--

Out 5.

Out 4.5. T" C 30. Additions Suspended MnSOs. Plating time 24 amp.hrs/sq. ft. cathode. Current density:

Anode amps. per sq. ft.

Cathode 75 amps. per. sq. ft. Deposit:

Character Smooth.

Efficiency 60%.

Adherence Strippable.

Anode behavior: Lead-silver anode formed a small amount of adherentoxide. Notes: Cathode stainless steel type 316.

Example 3 Type of cell Single compartment. Electrolyte:

Acid ion Chloride.

Mn g./l 12.

NH; salt g./l 125. Suspended solid:

Chemical nature MnCOa.

Physical nature 5 microns.

Amount g./1.

Out 1 Out 4.5.

T C 30. Additions Suspended M11503. Plating time 24 amp. hrs/sq. ft.cathode. Current density:

Anode 100.

Cathode 100. Deposit:

Character Smooth.

Efiiciency 70%.

Adherence Strippable.

Anode behavior: 1% ammonia loss at graphite anode. Notes: Flow rate 1liter per minute per sq. ft. cathode.

Cathode titanium.

Example 4 Example In a further repetition of Example 3 all of theconditions of the latter example were observed except that the suspended(solid) manganese carbonate was in very fine (about 1 micron)crystalline form. It was found the amount of suspended solid had to begreatly reduced-- 2 g./l. in, and 0.1 g./ l. out-in order to hold the pHof the outgoing electrolyte at 4.5. In this example a classifier wasinterposed in the circuit ahead of the cell. good, strippable depositwas obtained Example 6 Type of cell Single compartment. Electrolyte Acidion Fluoborate.

Mn g./l 12..

NH4 salt g./l 125.. Suspended solid:

Chemical nature Manganese carbonate.

Physical nature 5 microns.

Amount g./l.

Out 8...

Out 4.5.

T" C 30. Additions Suspension of .2 g./1 MnS. Plating time 2400 amp.hrs/sq. ft. Current density:

Anode 60.

Cathode 60.

Deposit:

Character Smooth.

Efficiency 60%.

Adherence Strippable.

Anode behavior: No attack or oxidation on graphite anode. Notes:Stainless steel cathode.

Example 7 The conditions were the same as in Example 3 except that thetemperature was 50C. The deposit was slightly nodular and very difiicultto strip, and the efliciency was 55%.

Example8 The conditions were identical with Example 3 except that thetemperature was maintained at 10 C. The

deposit was slightly nodularand was adherent and diificult to strip; theefficiency was 60%.

Example -9.EContinued T C 30. Additions Suspended sulphate. Currentdensity:

Anode 100. Cathode 50. Deposit:

Character Smooth. Efliciency 60%. Adherence Strippable. Anode behaviorGraphite anode.

Example 1 0 The same conditions were observed, and the same resultssecured, as in Example 3. The acidity of the solution after leaving thecell was adjusted to pH=6.2 by stirring with manganese carbonate, and0.1 g./l. H25 and 0.1 g./-filter aid were added. The solution wasfiltered. Ammonia was added to about 17 mol/liter, and CO2 to about 4mol/liter. The mixture was warmed to 70 C. for 30 minutes to precipitateCaCOa and MgCOs. The precipitate was removed by filtration, and thefiltrate was used to neutralize exit solution at pH=4.'5 to pH=6.2 forre -use.

Example 11 The procedure in this example was the same as in Example 3.In carrying out the process cyclically, I add BaClz to the exit solutionto precipitate any sulphate formed. A small excess of 'BaClz does noharm and may advantageously be maintained in the circuit so that anysulphate formed is precipitated. Precipitated sulphate must beperiodically removed by acidifying to pH=5.0 with HCl to dissolve MnCOs,and filtering.

The drawing shows an area wherein the electro-deposit is smooth and isstrippable from the cathode, and further shows an area wherein theelectro-deposit-while not nodular and quite smoothis firmly adherent tothe cathode. While this latter area is to be avoided when electrowinningmanganese, it may be taken advantage of in case the desired result iselectroplating with manganese. Examples 7 and 8 :illustrate suchelectroplating situations where a titanium sheet is made the cathode.Similarly, firmly adherent electroplates of manganese on other basemetals may be effected by purposefully maintaining the temperature ofthe electroplating operation at below 20 C. or above 40 C. and thecurrent density at not substantially in excess of amp. sq.-ft.

I claim:

1. In a process for producing electrolytic manganese by electrolysis ofan electrolyzable manganese-containing solution in an electrolytic cellhaving an anodic electrode and a cathodic electrode, the improvementwhich consists in having present in said electrolyzablemanganese-containing solution, in contact with a surface of an electrodeof the cell during the electrolysis .operation, a suspended finelydivided solid acid-soluble oxidic manganous compound in quantitysufficient to maintain the pH of the electrolyte at from about 3.0 vtoabout 6.0.

2. The improved process defined in claim 1, characterized in that thesuspended finely divided, solid, acidsoluble, oxidic manganous compoundis selected from the group consisting of manganous carbonate andmanganous oxide.

3. The process defined in claim 1,.in .which the electrolyte containingthe suspended .finelydivided solid oxidic manganous compound is passedover the surfaces of the anode and of the cathode in asinglecompartmentcell.

4. The process defined in claim 3, in whichtheelectrolyte suspension ispassed throu'ghthecell.-at.such.a.rate

that its initial content of suspended finely divided solid 9 oxidicmanganous compound is not completely exhausted as it exits from thecell.

5. In the process of electrodepositing manganese by electrolyzing anelectrolyzable solution of manganese and ammonium chlorides as the sameis passed through an electrolytic cell provided with a cathode and acarbon anode, the step of minimizing decomposition of ammonium chlorideadjacent the anode by maintaining in suspension in the electrolyte asthe same flows through the anode space of the cell in contact with theanode surface a finely divided, solid, acid-soluble oxidic manganouscompound in quantity sufficient to maintain the pH of the electrolyteadjacent the anode at above 3.0 and not more than 6.0 and operating atan anode current density below 150 amperes per square foot of anodesurface.

6. Process of electrodepositing manganese by electrolyzing anelectrolyzable solution of manganese and ammonium chlorides in anelectrolytic cell provided with a cathode and a carbon anode,characterized in that the electrolyte during electrolysis thereofcontains suspended therein a finely divided, solid, acid-soluble, oxidicmanganous compound and is passed through the cell between the cathodeand the anode at such a rate that its pH upon exiting from the cell ismore than 3.0 and less than 6.0 when the anode current density is lessthan 150 amperes per square foot of anode surface.

7. Process of electrodepositing manganese as defined in claim 6, inwhich the pH of the electrolyte as it enters the cell is within therange 6.0-6.5 while its pH as it leaves the cell is within the range4.0-6.0, and in which the temperature is maintained in the range -40 C.and the current density is at least amperes but not materially in excessof 130 amperes per square foot, the temperature and current densitybeing correlated to yield a non-adherent and non-nodular plating.

8. Process of electrowinning manganese which comprises suspending asubstantially pure and fully crystalline manganous carbonate in anelectrolyte containing 6-18 grams per liter of manganese as chloride and100-150 grams per liter of ammonium chloride and being saturated withmanganous sulphite, adjusting the pH of the resulting electrolyzablesuspension of manganese carbonate to -65 at 2030 C., passing theelectrolyzable suspension through an electrolytic cell having a carbonanode and a titanium cathode, and in contact with said anode andcathode, at such a rate that the electrolyzable suspension leaving thecell has a pH within the range 4.0-6.0 while passing a unidirectionalelectric current through the cell at a cathode current density of 40-125amperes per square foot of cathode surface, and maintaining thetemperature within the range 20-40 C.

9. Process of producing a firmly adherent electroplate of manganese on aconductive metal article which comprises making said metal article thecathode in a single compartment electrolytic cell, preparing anelectrolyte suspension by suspending in an electrolyzable solution ofmanganese and ammonium salts a finely divided particulate acid-solublemanganous carbonate in quantity sufiicient to maintain the pH of theelectrolyte at from about 3.0 to about 6.0, passing the resultingelectrolyte suspension through the cell between said cathode and acarbon anode, passing unidirectional current at a current density notsubstantially in excess of amperes per square foot through theelectrolyte suspension, so controlling the flow of the electrolytesuspension that it is partially neutralized and replenished in manganeseby dissolution of the particulate manganous carbonate in saidelectrolyte as the same flows through the cell and in contact with thecathode, and maintaining the electrolyte suspension at a temperaturewithin one of the ranges 10-20 C. and 40-50 C. and the current densityat from 40 amperes to not materially in excess of amperes per squarefoot, the temperature and current density being correlated to yield anadherent, non-nodular plating.

References Cited in the file of this patent UNITED STATES PATENTS2,119,560 Shelton June 7, 1938 2,317,153 Dean Apr. 20, 1943 2,356,515Guareschi Aug. 22, 1944 2,396,570 Guareschi Mar. 12, 1946 2,417,259Mitchell et al Mar. 11, 1947 2,546,547 Koster Mar. 27, 1951 2,608,531Fox Aug. 26, 1952

1. IN THE PROCESS FOR PRODUCING ELECTROLYTIC MANGANESE BY ELECTROLYSISOF A ELECTROLYZABLE MANGANSES-CONTAINING SOLUTION IN A ELECTROLYTIC CELLHAVING AN ANIODIC ELECTRODE AND A CATHODIC ELECTRODE, THE IMPROVEMENTWHICH CONSISTS IN HAVING PRESENT IN SAID ELECTROLYZABLEMANGANESE-CON-TAINING SOLUTION, IN CONTACT WITH A SURFACE OF ANELECTRODE OF THE CELL DURING THE ELECTROLYSIS OPERATION, A SUSPENDEDFINELY DIVIDED SOLID ACID-SOLUBLE OXIDIC MANAGANOUS COMPOUND IN QUANTITYSUFFICIENT TO MAINTAIN THE PH OF THE ELECTROLYTE AT FROM ABOUT 3.0 TOABOUT 6.0.